Dexter Morgan appears to have achieved a fragile balance in this season. He is now a married man, living in a suburban home with his wife, Rita, and her three children, including a newborn son, Harrison. The demands of family life constantly clash with his Dark Passenger, his compulsive need to kill. Dexter is exhausted, struggling to find time for his ritualistic murders while changing diapers and attending neighborhood barbecues.
His professional life at the Miami Metro Homicide department is equally strained. A major new case emerges: a mysterious killer the media dubs the “Trinity Killer” is discovered to have been operating undetected across the country for over thirty years, murdering victims in a ritualistic cycle of three. The FBI is called in, and the pressure to solve the case is immense.
Dexter becomes fascinated with Trinity, whose real name is Arthur Mitchell. Mitchell is a pillar of his community, a family man, and a devout churchgoer, presenting a perfect facade of normalcy. Dexter sees in Arthur a dark reflection of his own life, a man who also manages to balance a double life. He becomes convinced that by studying Arthur, he can learn how to sustain his own family and his secret simultaneously.
Ignoring all caution, Dexter begins to insinuate himself into Arthur Mitchell’s life. He stalks him, studies his methods, and even initiates a risky, direct relationship with him under a false identity. This obsession becomes increasingly dangerous, pulling Dexter away from his own family and blurring the lines between his hunt and a twisted form of mentorship.
Meanwhile, Dexter’s sister, Debra, is dealing with severe trauma from the events of the previous season and is pursuing a sergeant’s exam. Their relationship is tested as Dexter’s secrets and distractions grow. Sergeant Doakes is gone, but Agent Frank Lundy from the FBI returns to lead the Trinity investigation, renewing a complicated connection with Debra.
The season builds to a devastating climax. Just as Dexter finally corners Arthur and executes him according to his own code, believing he has triumphed and secured his family’s future, he returns home to a horrifying discovery. He finds his infant son, Harrison, sitting alone on the kitchen floor, crying in a pool of blood. In the bathtub lies his wife, Rita, murdered in the exact same manner as Arthur Mitchell’s first victim—a bathtub slaying. The cycle has completed. Trinity had the last word, killing Rita as a final act of vengeance and to teach Dexter a lesson about the consequences of his actions.
The season ends with Dexter holding his son amidst the horrific scene, realizing that his pursuit of a normal life and his failure to kill Arthur sooner has led to catastrophic loss. The facade of the perfect family is shattered forever, leaving him utterly alone with his darkness.